1. Field of the Invention
This invention relates generally to electronic circuits, and more particularly to circuits for the transmission of digital signals. Even more particularly, this invention relates to a scheme for terminating a clock network.
2. Background of the Invention
Modern digital electronic devices, such as computers, often have many different components that receive the same timing reference or clock signal. This makes communication between these components much simpler. One common clock distribution topology, used to reduce clock skew, and reduce cost, involves driving multiple transmission lines of equal length with a single device. At the end of each of these transmission lines is a receiving device, and a termination impedance. The termination impedance is matched to the intrinsic impedance of the transmission line to prevent, or reduce, reflections off the end of the transmission lines. This, in turn, reduces the amount of noise on a signal allowing the designer to specify smaller noise margins. Smaller noise margins allow signals to switch faster, improving the overall speed of the electronic device.
Unfortunately, this termination scheme suffers from several problems. First, each termination impedance at the end of a transmission line increases the DC current the driver must supply. This increases the power drawn by the electronic device. For example, if there are five 50 .OMEGA. terminations connected between ground and the end of five transmission lines, the driver must supply 100 milliamps to maintain the signal at 1 volt above ground. For a clock signal with a 50% duty cycle, the driver would draw an average of 50 milliwatts. In contrast, if each of the transmission lines has a 100 picosecond delay, and is unterminated, the driver would only draw an average of 330 microwatt for a 33 megahertz clock signal.
Another problem is the termination impedance. For each receiving device, a termination impedance is needed. To ensure reflections are minimized, this impedance must be calibrated against process variations to closely match the intrinsic impedance of the transmission line. That means that for five receiving devices, five calibration circuits must be built, or five passive impedances must be laser trimmed. This increases the cost of building the electronic device.
Accordingly, there is a need in the art for an improved termination scheme for clock networks that does not draw significant DC current. This scheme should minimize the number of devices that need to be matched, or calibrated. Furthermore, this scheme should be tolerant of moderate variations in the length of the transmission lines delivering the clock signal. These needs, and others, are met by the present invention.